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We study a one-unit repairable system, supported by two identical spare unitson cold standby, and serviced by two types of repairers. The model applies, forinstance, to ANSI (American National Standard Institute) centrifugal pumps in achemical plant. The failed unit undergoes repair either by an in-house repairerwithin a random or deterministic patience time, or else by a visiting expertrepairer. The expert repairs one or all failed units before leaving, and doesso faster but at a higher cost rate than the regular repairer. Four modelsarise depending on the number of repairs done by the expert and the nature ofthe patience time. We compare these models based on the limiting availability$A_{\infty}$, and the limiting profit per unit time $\omega$, using semi-Markovprocesses, when all distributions are exponential. As anticipated, to maximize$A_{\infty}$, the expert should repair all failed units. To maximize $\omega$,a suitably chosen deterministic patience time is better than a random patiencetime. Furthermore, given all cost parameters, we determine the optimum numberof repairs the expert should complete, and the optimum patience time given tothe regular repairer in order to maximize $\omega$.

A Repairable System Supported by Two Spare Units and Serviced by Two Types of Repairers